1. Field of the Invention
The present invention relates to a non-volatile memory control circuit for controlling erasure of non-volatile memory where source line voltage is manipulated in units of a sector composed of a predetermined plurality of memory cells.
2. Description of the Related Art
Heretofore, flash memory has become widely popular as non-volatile memory to which data can be rewritten. Data erasure in flash memory is accomplished by applying a high voltage HV to a word line (WL) and removing electrons held in a floating gate (FG) of the memory.
On the other hand, in a programming operation for writing data, a high voltage is applied to the source line. Since the high voltage is also applied to memory cells that are not to be programmed, electrons enter the floating gate (FG) in accordance with the total application time of the high voltage and may cause a transfer to a program state, namely, a program disturb may occur, such as due to reverse tunneling phenomenon. When the program disturb occurs, data cannot be normally retrieved during read operations.
Consequently, when rewriting data to flash memory, erasure is performed in sector units formed from a plurality of memory cells of the same source line after which programming is performed. As a result, both erasure and programming are performed at the same sector unit so as to prevent the non-retrieval of normal data during read operations due to the influence of program disturb.
Japanese Patent Laid-Open Publication No. Hei 11-328985 discloses the related art.
Here a sector is a relatively large unit resulting in a problem where an entire sector must be erased and a large amount of data must be exchanged to rewrite data even when it is desirable to erase only a small amount of data.
Furthermore, the life of flash memory is often determined by the number of erasures. There is also a demand to decrease the number of erasures in cellular telephones where the data rewrite frequency of flash memory is extremely high.